Method for determining the layer thickness of a connecting layer between two packaging layers

ABSTRACT

For a simple, fast, safe and reliable determination of the layer thickness of a bonding layer between two layers of a packaging, a laser ultrasonic method is provided, in which the transit time of the ultrasonic wave through the first and second packaging layers ( 2, 3 ) is determined in advance, and a maximum (M 1 , M 2 , M n ) in the measurement signal (S) is sought, and the point in time of occurrence of this maximum (M 1 , M 2 , M n ) is determined as the total transit time (T 1 , T 2 , T n ) of the ultrasonic wave, and the transit time of the ultrasonic wave through the first and second packaging layers ( 2, 3 ) is subtracted from the total transit time (T 1 , T 2 , T n ), and the thickness (d) of the bonding layer is deduced from the known ultrasonic speed (v S ) in the bonding layer ( 5 ).

The present invention relates to a method for determining the layerthickness of a bonding layer between a first packaging layer and asecond packaging layer, wherein the first layer is acted upon by a laserpulse and the resulting ultrasonic wave is detected on a surface of thefirst or second packaging layer, and the measurement signal, detectedover time, is evaluated to determine the thickness of the bonding layer.

Packaging for foods or animal feed today often consists of containers,for example of aluminum or a plastic laminate, and a cover film made ofplastic, aluminum, paper or a laminate thereof, for example for sealingthe container. Alternatively, bags can also be produced from a film or afilm laminate material and sealed by a seam after being filled. Asealing process, for example a heat sealing process or an ultrasonicsealing process, or an adhesive bonding process is often used for thesealing. A sealing medium such as polyethylene, polypropylene and/orcopolymers thereof is applied, for example by a coextrusion process toboth the container and also the cover film and/or to both sheets offilm. In the case of adhesive bonding, it is sufficient if an adhesiveis applied to only one portion, and naturally the adhesive can also beapplied to both portions.

In contrast with the thicknesses of the cover film and of the container,which can be kept in a narrow tolerance range during the manufacturingprocess and is thus more or less constant, the resulting thickness ofthe sealing layer or adhesive layer is subject to great fluctuations dueto the process. Therefore, in manufacturing such packaging containers,the thickness of the sealing layer or the adhesive layer must bemonitored continuously as part of quality assurance. This is currentlyaccomplished by means of a mechanical gauge, for example. To do so,however, the sample must first be etched by using an acid to expose thelayer to be measured and to enable measurement of the thickness of thesealing layer and/or the adhesive layer. However, such methods arecomplex, because it is necessary to work with acid and great safetymeasures must be taken and observed with regard to occupational safety.Furthermore, this method is time consuming because, on the one hand, theworking steps require time and, on the other hand, a great deal ofmanual labor is involved. The layer thickness can also be determined bymeans of computer-assisted tomography, but that is expensive due to thecost of the equipment.

In addition, there are also known noncontact methods based on laserultrasonic spectroscopy. For these methods, a sample is treated with ashort laser pulse and thus with a broad frequency spectrum. Apropagating ultrasonic wave is created in the sample due to athermoelastic heating and/or ablation of the surface of the sample, andthis is also detected in a noncontact process, for example by means ofan interference method, for example using a laser Doppler vibrometer.Detection can be performed in the vicinity of the excitation or on anopposing surface of the sample. By evaluating the frequency spectrum ofthe detected signal, it is possible to deduce certain properties of thesample, for example, its layer thickness. Such a method is described inCA 2 314 305 A1 for a multilayer sample, for example. This requires amodel that describes the behavior of the multilayer sample and includesat least one parameter that is sought, such as the thickness of a layerfor example, which influences the resonant frequency. The at least oneparameter is then varied by using a best fit method to obtain the bestpossible correspondence of the resonant frequencies predicted by themodel with the frequencies actually measured. The quality of this methoddefinitely depends on the model of the sample, with a suitable modelbeing required for each sample and therefore a very great effort beingrequired. In addition, the method also involves a very highcomputational effort, in particular when several parameters are unknownand are to be determined. In the case of multiple unknown parameters,the uncertainty of the measurement method also increases and, along withthat, the standard deviation of each parameter to be fitted alsoincreases.

One object of the present invention was therefore to determine the layerthickness of a bonding layer between two layers of a packaging by asimple, safe, fast and reliable method.

This object is achieved according to the invention by determining thetransit time of the ultrasonic wave through the first and second layersof packaging in advance, and by searching a maximum in the measurementsignal and the point in time of occurrence of this maximum is determinedas the total transit time of the ultrasonic wave, and then the transittime of the ultrasonic wave through the first and second packaginglayers is subtracted from the total transit time, and the thickness ofthe bonding layer is deduced from the known ultrasonic velocity in thebonding layer. By using available a priori knowledge, the thickness ofthe bonding layer can be determined in a very simple and rapid mannerwithout having to perform complex signal analyses and calculations. Thisalso allows measurements to be performed on a great many measurementpoints in a short period of time.

The search for the maximum can take place by cross-correlation with aknown signal pattern in a first method or by means of a priori knowledgein a second method, when a time window is defined in the range of thetotal transit time and the maximum is sought within this time window.

Without a priori knowledge, the search for the first local maximum maybe conducted in a third method by analyzing the noise in the spectrumand the noise power in the measurement signal before the first signalarrival to be expected in order to find a threshold value in theamplitude and rise time. With the help of these threshold values, thefirst local maximum can be determined. This third method includescertain error sources, such as voltage peaks, which are caused byhigh-frequency scattering, for example, and can be misinterpreted, butit has the great advantage of allowing a search for a maximum without apriori knowledge.

To compensate for statistical fluctuations in measured values, it ispossible to provide for a plurality of individual measurements to beperformed within a measurement grid and for averages to be formed overthese individual measurements to determine the thickness of the bondinglayer. The required individual measurements can be performed in a veryshort period of time by means of the measurement method according to theinvention.

To be able to correctly detect regions of a different surface structure,for example embossing in a packaging layer, a histogram of therespective transit times can be prepared from the maximums determinedfrom the individual measurements and then can be used to ascertainregions of a different surface structure. The thickness of the bondinglayer can then be determined from this graph for at least one surfacestructure detected.

The present invention is explained in greater detail below withreference to FIGS. 1 through 5, which show advantageous embodiments ofthe invention in an exemplary, schematic and nonrestrictive form,showing:

FIG. 1 a schematic measurement arrangement according to the invention,

FIG. 2 a recorded measurement signal of the ultrasonic wave generated,

FIG. 3 the measurement at various measurement locations, each with aplurality of measurement points, and

FIGS. 4 and 5 the measurement and evaluation on a structured packaginglayer.

FIG. 1 shows schematically and in greatly simplified form a setup fordetermining the thickness of a bonding layer on a package 1. The packageconsists of a first packaging layer 2, for example a container, such asa dish, and a second packaging layer 3, for example a cover film. Abordering region 4 is provided here on a peripheral border of the firstpackaging layer 2, so that the second packaging layer 3 can be attachedto the first packaging layer 2 at this border.

However, the package could of course also be embodied in the form of abag or something similar, connected to one another in the region of aconnecting seam. The first packaging layer 2 may be produced, forexample from paper, aluminum or a plastic or a laminate of thesematerials in the form of a multilayer film. The second packaging layer 3may be produced from paper, aluminum or a plastic or a laminate of thesematerials in the form of a multilayer film. The first and/or secondpackaging layer 2, 3 may also be printed on one side, preferably thefree visible surface, and/or may be metallized on one side, preferablythe side facing the interior of the package 1.

A bonding layer 5, for example a sealing layer or an adhesive layer, isprovided for bonding the two packaging layers 2, 3. To measure thethickness d of the bonding layer 5, one side of the second packaginglayer 3 is excited with an excitation laser 10 by means of a short laserpulse 11, preferably with a pulse duration of <1 ns. Due to the shortlaser pulse 11, a broadband ultrasonic wave is generated in the package,which propagates in the package 1. After passing through the secondpackaging layer 3, the bonding layer 5 and the first packaging layer 2,this ultrasonic wave is detected on the surface of the first packaginglayer 2, i.e. opposite the excitation, using a detector 12. However, thedetector 12 may also be arranged so that it detects the reflectedultrasonic wave on the surface of the second packaging layer 3, i.e. onthe same side as the excitation.

The detector 12 may be embodied as an interferometer, for example as aknown Fabry-Perot interferometer, as a homodyne or heterodyne Michelsonor Mach-Zehnder interferometer, as a photorefractive interferometer orthe like. Using this interferometer, the vibration of the surface of thefirst or second packaging layers 2, 3 induced by the ultrasonic wave issensed and detected. The design, the components and the function of suchinterferometers have long been adequately well known, so that they neednot be discussed in detail here. The detector 12 supplies a measurementsignal S to an evaluation unit 13.

The measurement signal S recorded in this way is depicted in FIG. 2, forexample, where the lower curve corresponds to the crude signal and theupper curve corresponds to the crude signal that has been filteredand/or processed in a band pass filter, for example. One can recognizein the measurement signal S distinct maxima M_(n), namely a firstmaximum M₁ after approx. 20 ns and other following, attenuated maximaM₂, M_(n), which are caused by the echo of the ultrasonic wave in thepackaging 1. To determine the thickness d of the bonding layer 5, thetransit times T of these maxima M_(n) are evaluated. The first maximumM₁ with the strongest signal is preferably used, but any other maximummay also be used.

To do so, the transit times of the ultrasonic wave in the first andsecond packaging layers 2, 3 are determined in advance. This is done bymeans of reference measurements or by calculation from the knownphysical properties of the material. It is then known how long theultrasonic wave needs to pass through the packaging layers 2, 3. Sincethe packaging layers can be produced with a very high precision andconstancy with regard to the thickness as well as the properties of thematerial, they can therefore be assumed to be more or less constant. Thetotal transit time T₁ of the ultrasonic wave through the packaging 1must therefore be determined at the location to be measured.

Therefore, the point in time of occurrence of a maximum M, preferablythe first maximum M₁, is sought in the measurement signal S thatcorresponds to the total transit time T₁ of the ultrasonic wave throughthe packaging 1.

Known methods are recommended for the search for the maximum such as,for example, cross-correlation with known signal patterns (state of theart in radar applications, GPS, ultrasound, etc.), a search for themaximum within certain limits or a search for the first local maximum byanalyzing the noise in the spectrum and the noise power in themeasurement signal S before the expected arrival of the first signal inorder to find a threshold value in the amplitude and rise time. With thehelp of these threshold values, the first local maximums can bedetermined without a priori knowledge of the measurement signal S.

The known transit times T₂, T₃ of the first and second packaging layers2, 3 are subtracted from the total transit time T₁, so that the transittime T_(S) through the bonding layer 5 remains. Then the thickness d ofthe bonding layer 5 can be deduced from this transit time T_(S). Thepropagation speed v_(S) of the ultrasonic wave through the material ofthe bonding layer 5 is either known or can be determined from referencemeasurements. The thickness d of the bonding layer 5 can then becalculated directly from the determined transit time T_(S), for example,in the form d=T_(S)−v_(S), where T_(S)=T₁-T₂-T₃. In other words, todetermine the thickness d of the bonding layer 5, a priori knowledgeabout the packaging 1 is relied on.

However, this a priori knowledge may also be used to simplify the searchfor the maximum and thus to accelerate the determination of thethickness d of the bonding layer 5. For example, a time window Z withinwhich the occurrence of a maximum is to be expected (see FIG. 2) can bedefined on the basis of the transit times T₂, T₃ in the first and secondpackaging layers 2, 3, which can be regarded as known, see FIG. 2, andthe search for a maximum, for example by means of cross-correlation, canthen be limited to this time window Z. Since the approximate thickness dof the bonding layer 5 and/or a range of the possible thickness d isknown from the production process, the time window Z can be determinedeasily. The search for the maximum can then be limited to this timewindow Z.

Without a priori knowledge, the search for the first local maximum maytake place by analyzing the noise in the spectrum and the noise power inthe measurement signal before the first signal arrival to be expected(see FIG. 2, t<10 ns), in order to find a threshold value in theamplitude and rise time. The first local maximums M₁ can be determinedwith the help of these threshold values.

An improvement in the evaluation can also be achieved if thedetermination of the thickness d of the bonding layer 5 is not made onthe basis of a single measurement but instead is made on the basis of aplurality of measurements, as shown in FIG. 3. In the range of thebonding layer 5, i.e., here in the bordering region 4 of the containerof the package 1, for example, a number of measurement regions 20, forexample a region of 1.5 mm×1.5 mm each, is defined. Several measurementpoints 21 are provided in each measurement range and are arranged in ameasurement grid with a distance of 50 μm between the measurement points21, for example. The local thickness of the bonding layer 5 isdetermined at each measurement point 21, and the individual localthicknesses are then averaged to determine the thickness d of thebonding layer 5 for the respective measurement region 20.

To do so, it is possible to provide that the package 1 is arrangedmovably in a plane normal to the excitation laser beam, for examplebeing clamped on a carriage movable in this plane by means of drivenlinear guides. The packaging 1 is then positioned accordingly for theindividual measurements by means of a suitable control, for examplebeing integrated into the evaluation unit 13. Alternatively oradditionally, the excitation laser 10 and the detector 12 may also bearranged movably. Based on the use of high-energy lasers, the devicemust of course also satisfy the safety provisions that are provided.

It is also possible that a packaging layer 3 has a surface structure,for example an embossing, as represented in FIG. 4. Based on thissurface structure, the local thicknesses d of the bonding layer 5 mayfluctuate. By averaging as described above, a false thickness d of thebonding layer 5 would then result. To prevent this, the total transittimes T₁ (or the transit time T_(S) through the bonding layer 5) areplotted in a histogram from the individual measurements at themeasurement points 21, as depicted in FIG. 5. To do so, as is wellknown, intervals of the total transit times T₁ are determined and thefrequency H is determined, indicating how many measurements fall in therespective intervals. Then regions of different surface structure can bedetermined from the histogram, a range of transit times belonging toeach region. Either an average transit time T₁₁, T₁₂ for each region isdetermined from this and then the thickness d of the bonding layer 5 forthe regions of different surface structure can be determined from this,or the individual measurements are assigned to different regions andthen an average value is determined as described above.

The invention claimed is:
 1. A method for determining the layerthickness of a bonding layer between a first packaging layer and asecond packaging layer, the method comprising: acting on the firstpackaging layer with a laser pulse, resulting in an ultrasonic wave at asurface of the first and second packaging layers; detecting theresulting ultrasonic wave at the surface of one of the first or secondpackaging layer; and evaluating a measurement signal, which is detectedover time, for determining the bonding layer thickness, wherein apredetermined transit time of the ultrasonic wave through the first andsecond packaging layers is known prior to the evaluating of themeasurement signal, the evaluation of the measurement signal comprising:looking for an occurrence of a maximum in the measurement signal;determining, from a point in time of a found occurrence of the maximumfound in the measurement signal, the total transit time of theultrasonic wave; subtracting the predetermined transit time of theultrasonic wave through the first and second packaging layers from thetotal transit time; and deducing, from a known ultrasonic speed in thebonding layer, the thickness of the bonding layer.
 2. The methodaccording to claim 1, wherein the evaluating further comprises defininga time window in a range of the total transit time within which thelooking for the maximum occurs.
 3. The method according to claim 1,wherein the maximum is determined by cross-correlation with a knownsignal pattern.
 4. The method according to claim 1, wherein the noise inthe spectrum and the noise power are analyzed in the measurement signalbefore the first expected signal arrival, in order to find a thresholdvalue in the amplitude and rise time as an indication of the maximum. 5.The method according to claim 1, wherein the evaluating of themeasurement signal further comprises: performing a plurality ofindividual measurements within a measurement region and averaging theplurality of individual measurements to determine the bonding layerthickness.
 6. The method according to claim 5, wherein regions of adifferent surface structure are determined from a histogram ofrespective total transit times of the plurality of individualmeasurements prepared from respective found maxima of the individualmeasurements, whereby a thickness for at least one surface structure isascertained.
 7. A method for determining the layer thickness of abonding layer between a first packaging layer and a second packaginglayer, the method comprising: acting on the first packaging layer with alaser pulse, resulting in an ultrasonic wave at a surface of the firstand second packaging layers; detecting the resulting ultrasonic wave atthe surface of one of the first or second packaging layer; andevaluating a measurement signal, which is detected over time, fordetermining the bonding layer thickness, wherein a predetermined transittime of the ultrasonic wave through the first and second packaginglayers is known prior to the evaluating of the measurement signal, theevaluation of the measurement signal comprising: looking for anoccurrence of a maximum in the measurement signal; determining, from apoint in time of a found occurrence of the maximum found in themeasurement signal, the total transit time of the ultrasonic wave;subtracting the predetermined transit time of the ultrasonic wavethrough the first and second packaging layers from the total transittime; and deducing, from a known ultrasonic speed in the bonding layer,the thickness of the bonding layer, wherein the evaluating of themeasurement signal further comprises: performing a plurality ofindividual measurements within a measurement region, and wherein regionsof a different surface structure are determined from a histogram ofrespective total transit times of the plurality of individualmeasurements prepared from respective found maxima of the individualmeasurements, whereby a thickness for at least one surface structure isascertained.